Re: Fix bug in multixact Oldest*MXactId initialization and access

(added back pgsql-hackers, I think you replied in private by accident)

On 28/02/2026 00:07, Sami Imseih wrote:
> 1/
>
> +MyOldestMemberMXactIdSlot(void)
> +{
> + Assert(MyProcNumber >= 0 && MyProcNumber < MaxBackends);
>
> It would be better to use NumVisibleSlots instead of MaxBackends in
> the second assert condition.

MaxBackends makes the assertion more strict. It verifies that we use one
of the slots reserved for regular backends, not prepared xacts. I'll add
a comment on that.

> 2/
>
> +static inline MultiXactId *
> +PreparedXactOldestMemberMXactIdSlot(ProcNumber procno)
> +{
> + Assert(procno >= FIRST_PREPARED_XACT_PROC_NUMBER);
> + Assert(procno - FIRST_PREPARED_XACT_PROC_NUMBER <
> + NumMemberSlots);
> + return &OldestMemberMXactId[procno -
> + FIRST_PREPARED_XACT_PROC_NUMBER];
> +}
>
> given
>
> +#define FIRST_PREPARED_XACT_PROC_NUMBER \
> + (MaxBackends + NUM_AUXILIARY_PROCS)
>
> let's say, MaxBackends = 100 and NUM_AUXILIARY_PROCS = 38, the
> first prepared transaction procno will be 138. The current math
> will return an index of 0 (138 - 138 = 0), but this corrupts
> backend slot 0.

Oof, you're right.

> Should we not account for NumVisibleSlots to skip
> over the regular backend slots?
>
> ```
> return &OldestMemberMXactId[NumVisibleSlots + (procno -
> FIRST_PREPARED_XACT_PROC_NUMBER)];
> ```

That's not quite right either. NumVisibleSlots has nothing to do with
the OldestMemberMXactId array, MaxBackends is the right offset here.
NumVisibleSlots == MaxBackends, but that's just a coincidence.

New version attached.

- Heikki